First of all, the prior art shall be explained with reference to an example of an apparatus for the automatic measurement of the eye's refractive power.
In conventional automatic eye refractive power measuring apparatus, various improvements have be made to simplify the mechanism, to facilitate its production and adjustment, to reduce the measuring time and to increase the measuring precision. Thus, there is provided an automatic eye refractive power measuring apparatus shown in the Japanese Official Gazette under Laid Open patent application No. 160538/1980, corresponding to U.S. Pat. No. 4,353,625.
Defects, however, exist in this automatic eye refractive power measuring apparatus, in that, as an image rotating prism is required to rotate beams, it is difficult, in the manufacture and adjustment, to rotate the image rotating prism around the optical axis as a center. The amount of light will attenuate due to the reflection of the image rotating prism and the measuring time can not be significantly reduced.
Therefore, in order to eliminate such defects, an automatic eye refractive power measuring apparatus shown in the Japanese Official Gazette under Laid Open patent application No. 165735/1982 corresponding to U.S. Pat. No. 4,526,451 has been provided.
In this latter apparatus, the eye to be inspected is periodically scanned alternatively in two known directions using slit-shaped illuminating beams at the same angle of inclination to the scanning direction. The reflected light from the two beams is received by two pairs of photoelectric converting elements and the phase difference of output signals between the photoelectric converting elements forming the respective pairs is employed to measure the refractive power of the inspected eye.
For a shadow projecting device, in order to make slit-shaped illuminating beams scan alternatively in two known directions, a light source, a prism and a rotary copper is provided as seen in FIGS. 16 and 17. The rotary chopper 113 is provided with slit apertures 114 all formed at the same angle of inclination to the rotating direction X, FIG. 16 is a developed view of the rotary chopper 113.
If it is assumed that there is no astigmatism in the eye to be inspected, and as in FIG. 17(a) the slit-shaped illuminating beams are made to scan the eye in one direction or as in FIG. 17(b) the slit-shaped illuminating beams are made to scan in the other direction, the light reflected from the eye of each of the slit-shaped illuminating beams will run respectively in the direction indicated by the arrow V or W through two pairs of photoelectric converting elements 120, 122 and 121, 123 onto the light receiving surface of a photoelectric coverter 136.
If it is assumed that there is an astigmatism in the eye being inspected, the above mentioned reflected lights will deviate by an angle (which shall be hereinafter called a column axis angle) corresponding to the direction of the main warp line of the astigmatism as compared with the direction shown on FIGS. 17(a) and 17(b).
Therefore, if S represents a spherical refractive power and C represents a columnar refractive power, and the slit-shaped illuminating beams are made to scan in the direction corresponding to FIG. 17(a), the value D.sub.1 obtained from the phase difference of the output signals of the photoelectric converting elements 120 and 122 will be EQU D.sub.1 =S+C cos.sup.2 .theta. (1)
and the value D.sub.2 obtained from the phase difference of the output signals of the photoelectric converting elements 121 and 123 will be EQU D.sub.2 =C/2sin 2 .theta. (2)
When, however, slit-shaped illuminating beams are made to scan in the direction corresponding to FIG. 17(b), the value D.sub.3 obtained from the phase difference of the output signals of the photoelectric converting elements 120 and 122 will be EQU D.sub.3 =-C/2sin2 .theta. (3)
and the value D.sub.4 obtained from the phase difference of the output signals of the photoelectric converting elements 121 and 123 will be EQU D.sub.4 =S+C sin.sup.2 .theta. (4)
Therefore, from the above formulae (1) to (4), the spherical refractive power S, columnar refractive power C and column axis angle .theta., that is, the refractive power can be determined.
However, in the automatic eye refractive power measuring apparatus shown in the above mentioned application, Japanese Publication No. 165735/1982, the slit-shaped illuminating beams must be made to scan alternatively to two known directions. Therefore, instead of using an image rotating prism, a prism which is not required in the Japanese Publication No. 160538/1980 is required. Consequently the effectiveness to the amount of light is not so high, and two light sources are required instead of the one light source required in the Japanese Publication No. 160538/1980--thus, a new defect in the manufacture is created in that the beams and light of the two light sources must be made to coincide respectively with each other.
Incidentally, as the eye is also an optical system, it is apparent that the above described argument is not limited to apparatus for the automatic measuring of eye refractive power but can be also used in such other refractive power measuring optical systems as a lens meter ad a radius of curvature measuring apparatus.
That is to say, the same defect as is described above is produced also, for example, in a lense meter such as described in Japanese Laid Open Patent application No. 168137/1982 to which the above mentioned Japanese Application No. 165735/1982 is applied and a radius of curvature radius measuring apparatus (such as in Japanese Laid Open Patent Application No. 197405/1982).